Field of the Invention
The invention provides a nanocomposite of multilayer fullerenes with transition metal oxide nanoparticles and a process for the preparation thereof. Particularly, the present invention relates to nanocomposite of multilayer fullerenes with transition metal oxide nanoparticles with enhanced specific capacitance. More particularly, this invention relates to a simple, cheaper and green approach for synthesis of activated multilayer graphitized fullerene in the form of nano-onions (CNOs) at low temperature from the pure ghee (clarified butter) for fabricating high performance exohedral type of super capacitors by incorporating suitable transition metal oxide.
Further the invention relates to supercapacitor composite electrode, comprising CNOs and transition metal oxide (MnO2) nanoparticles with enhanced specific capacitance.
Description of the Related Art
Carbon nanostructures with excellent properties such as mechanical, thermal, electronic, and electrical properties, coupled with chemical robustness, have spurred a broad range of applications. Various stable forms of structural organization of carbon atoms can exist on the nanoscale that includes zero dimensional, one-dimensional, and two-dimensional carbon nanomaterials viz. carbon nanotubes (CNTs), fullerenes, graphene, carbon quantum dots, nanodiamond and carbon nanoonions (CNOs) etc (D. Jariwala, et al. Chem. Soc. Rev., 2013, 42, 2824). With the recent advances in synthesis, fabrication and assembly techniques, carbon nanomaterials are experiencing renewed interest from researchers as a basis of numerous scalable technologies especially in the energy conversion and energy storage fields e.g. electrochemical capacitors.
Electrochemical capacitors, also called supercapacitors, store energy using an accumulation of ions of opposite charge in a double layer at an electrochemically stable, highly specific surface area electrode (J. R. Miller and P. Simon, Science, 2008, 321, 651). The electrochemical capacitors are considered to be very attractive energy storage devices, as they have high power density and cycling stability and are used to power hybrid electric vehicles, portable electronic equipment and other devices.
ESC stores charges by non-faradic capacitive process associated with the interfaces of solution and large surface area of active material and pseudocapacitive faradic redox reactions. Typically involves no change in chemical phase and composition and can be used for thousands of cycles without significant loss of capacity (Proceedings of the National Academy of Sciences, 2007, 104, 13574-13577).
Carbon nanomaterials have been extensively researched, either in an electric double layer capacitor as the carbon electrode or in pseudocapacitors that employ a composite of highly conducting carbon with a material such as a metal oxide. Porous activated, templated and carbide-derived carbons, multi- and single-walled carbon nanotubes and multilayer graphene have been used as the electrode materials in supercapacitors. A significant portion of the research is concentrated on CNTs and graphene based composite electrodes due to their excellent electrical and mechanical properties, chemical stability and high specific surface area. Capacitance as high as 200-400 F/g can be obtained from composites of graphene/CNTs with pseudocapacitive materials like metal oxides (Chen, J. Zhu et al. ACS Nano, 2010, 4, 2822) or polymers (ACS Nano, 2012, 6, 5941).
It is noteworthy, that the electrochemical performance of ESCs is greatly depend on the electroactive materials such as double layer capacitive carbon allotropes and pseudocapacitive electroactive oxides or hydrous oxides like MnO2, RuO2 and sulfide materials like ZnS and CoS and conductive polymers etc.
Inorganic pseudocapacitive materials such as MnO2, RuO2 store charge via redox reactions. Such inorganic electrochemical capacitors use an aqueous electrolyte to reduce the risk of explosion at high temperatures, improve the power density, and show higher efficiency than organic based supercapacitors.
The composite of supercapacitor comprising carbonaceous material and transition metal oxide pseudocapacitor material is reported in some state of arts.
WO2013070989 (Xie Ming et al.) discloses supercapacitor comprising at least two electrodes, each electrode being in electrical contact with an electrically conductive current collector, wherein at least one of the electrodes is a composite of a porous carbonaceous material, having a metal oxide pseudocapacitor material deposited thereon via an atomic layer deposition process.
WO2014062133 (Hui Ying Yang et al.) discloses a micro supercapacitor, comprising: a substrate; a first metal electrode; a second metal electrode; an active material coating the first metal electrode and the second metal electrode, comprising manganese oxide (MnO2), carbon nanostructures and optionally a binder; and an electrolyte.
Further High-performance flexible solid-state supercapacitors based on MnO2-decorated nanocarbon electrodes (CNT/CNO) films using the [EMIM][NTf2]-PVdF(HFP) gel electrolyte is reported in RSC Adv. 11 Sep. 2013, 3, 20613 by Yang Gao, et al.
The synthesis of a solid-state supercapacitor based on a CNPs/MnO2 nanorod hybrid structure through a simple flame synthesis method and electrochemical deposition process is disclosed in ACSNANO, review Oct. 26, 2011. Ruthenium Oxide (RuO2) is one of the highest specific pseudo-capacitive electrode materials among the transition metals along with high chemical and thermal stability and good electrochemical redox properties. Although it offers higher capacitance, high cost and the rarity of the material it was added to cheapest carbon electrochemical double layer electrode materials. Amorphous hydrous RuO2 will show higher specific capacitance than that of the crystalline as the intergranular water surface structure involves proton transport where as the oxide allows electronic conduction (Electrochimica Acta, 2006, 52, 1742-1748).
Tung-Feng Hsieh in Carbon (2012) 50, 1740-1747 discloses preparation of composite electrodes vertically aligned multi-walled carbon nanotubes (MWCNTs) coated with hydrous ruthenium dioxide (RuO2/nH2O), wherein the specific capacitance when using RuO2 nH2O/MWCNT/Ti as electrodes in 1.0 M H2SO4 aqueous solution can reach up to 1652 F/g at a scan rate of 10 mV/s.
Further Rituraj Borgohain et al. (J. Phys. Chem. C, 2012, 116 (28), pp 15068-15075) describes electrochemical study of functionalized carbon nano-onions for high-performance supercapacitor electrodes, wherein composite having RuO2 has specific capacitance of 334 F/g at a potential sweep rate of 20 mV/s with high power (242.8 kW/kg) and high energy density (11.6 Wh/kg).
“Microwave-polyol synthesis of nanocrystalline ruthenium oxide nanoparticles on carbon nanotubes for electrochemical capacitors” is reported in Electrochimica Acta Volume 55, Issue 27, 30 Nov. 2010, Pages 8056-8061 Ji-Young Kim et al. The specific capacitance was 450 F/g of ruthenium oxide/CNT composite electrode with 70 wt % ruthenium oxide at the potential scan rate of 10 mV/s. Further synthesis of carbon nano-onion and nickel hydroxide/oxide composites as supercapacitor electrodes” by Marta E. Plonska-Brzezinska et al. is reported in RSC Adv., 15 Oct. 2013,3, 25891-25901 (1225.2 F/g for CNOs/Ni(OH)2 and 290.6 F/g for CNOs/NiO, both at 5 mV/s).
Although considerable effort has been developed to enhance device performance so far, inefficient ionic and electronic transport in pseudocapacitive electrodes has led to capacitance fading over cycling or at high rates. In fact ionic and electronic transport kinetics are extremely important aspects in energy storage systems and that implies the critical significance of the electrode material's structure and morphology in achieving high power and energy densities, long cycle life, and high rate capability.
In case of nanoporous carbon supercapacitors counterions enter pores to form endohedral supercapacitors, which have a negative surface curvature. Graphene based materials have zero curvature, and one dimensional capped CNTs have a positive surface curvature, whereas, counterions can only reside on the outer surfaces, leading to exohedral supercapacitors (Chem. Eur. J., 2008, 14, 6614). These positive surface curvature carbon electrodes can be charged at a high rate, approaching electrolytic capacitors.
There are few methods known in the art for the production of Carbon Nano-Onions (CNOs) can be by the annealing of detonation nanodiamond powders in an inert atmosphere at temperatures above 1400° C. which leads to their graphitization and formation of CNO. However the efficiency of this technique is limited because of the unmanageable reaction, complex equipment, and high cost. The CNOs obtained from the nanodiamond soot showed to have specific electrochemical capacitance in the range of 20-40 F/g and further functionalization with polyaniline improved it to 640 F/g.
Therefore, there is need for efficient source of CNOs for fabricating high performance supercapacitors than conventional graphitic/mesoporous/activated carbons. Accordingly, the inventors have developed simple, cheaper, less complicated and green approach for the synthesis of CNOs using pure ghee (clarified butter) and comparatively at low temperature, 800° C., as an electrode for fabricating high performance electrochemical supercapacitors in presence of transition metal oxide.